What Is Flow Cytometry?

Method to Identify and Count Cells

Flow cytometry isn't a test for a specific disease or condition. Instead, flow cytometry is a technique that can used for a number of different purposes. A liquid containing cells or bacteria is sent through a very narrow tube. That allows properties of individual cells to be measured using lasers or other types of light. In medical settings, it is sometimes used for cancer diagnosis and treatment. It can also be used to check on patient health after organ transplant. What makes flow cytometry a useful technique is that it can be used to look at a large number of cells, one cell at a time.

CD3/CD4 test
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To better understand how flow cytometry works, imagine that you want to know the number of red candies in a bag. If you just look at the bag, you might see that some candies were red. However, there would be no way to know the exact number. How much easier would it be if someone put the candies on a conveyor belt so that they went past you one at a time? Then you could just count the red candies as you saw them. That's basically how flow cytometry works. Instead of trying to find a specific group of cells in a solution where they're all mixed together, they go past a detector one at a time.

Purpose of Test

Flow cytometry is used in many areas of clinical testing. That's because it's a relatively straightforward way to look for specific types of cells. Those cells can be cancer cells, immune cells, or even different types of sperm. As long as there is a way to mark cells for detection, flow cytometry can be used to find them. Usually, all it takes to identify a specific type of cell is to create a monoclonal antibody to recognize that cell. Then a fluorescent dye is attached to the antibody, and flow cytometry can find all the cells that the antibody targets.

Common uses for flow cytometry tests include:

  • Counting the number of CD4 T-cells in the blood of someone with HIV. This helps to determine how healthy their immune system is and to track any damage caused by the virus. This is generally done on a blood sample.
  • Counting the number of reticulocytes (immature red blood cells) in the bone marrow. This can be used to determine the cause of anemia. It can also be used to check the health of the bone marrow after a transplant or after chemotherapy. This is generally done on a bone marrow sample.
  • Histocompatibility (HLA) testing prior to an organ transplant, whether someone is intended to be a donor or a recipient. This is generally done on a blood sample.
  • Checking the number of sperm in a semen sample. This may be done either as part of an infertility workup or to see if a vasectomy has been successful.
  • Diagnosing and classifying leukemia or lymphoma. This may require a blood sample, bone marrow, or a different type of tissue sample.
  • Making certain your platelets, part of your clotting system, are working correctly.

Your doctor might order a flow cytometry test for any of a number of reasons. The specific test ordered will depend on what the doctor wants to know. The same thing is true for what type of sample the test will be run on.

Although flow cytometry tests can be useful in a variety of situations, they're not perfect. Since these tests are counting numbers of cells at a specific time, anything that alters those numbers has the potential to alter test results. For example, the presence of inflammation can alter the numbers of specific types of blood cells in a sample leading to a false negative or false positive test result for certain types of cancer . Similarly, a recent sexual experience can make a semen analysis inaccurate and taking a non-steroidal anti-inflammatory drug (NSAID) can affect a platelet function test.

Depending on the reason you are undergoing flow cytometry, your doctor may also order other tests. For example, when diagnosing certain types of cancer, your doctor may also want histochemistry.

Risks and Contraindications

The risks associated with flow cytometry are limited the risks associated with sample collection. Therefore, the risks depend on the type of sample being used. A blood sample or semen sample is relatively easy to collect safely. In contrast, a bone marrow sample or tissue sample is more difficult, and carries some additional risks. However, these tests are still considered to be quite safe. Significant amounts of bruising, bleeding, infection, and/or reactions to anesthesia are uncommon, but they are the most likely risks of bone marrow aspiration and/or tissue biopsy.

In general, doctors would not order a bone marrow biopsy or tissue sample if they did not suspect that they needed to analyze that type of sample to diagnose your illness. In fact, sometimes doctors will start with flow cytometry on a blood sample before moving on to more invasive testing. There are no specific contraindications to flow cytometry, although some people may need alterations to sample collection protocols.

Before the Test

What happens when your doctor decides you need a flow cytometry test will depend on the reason the test is needed. Sometimes, you will need to stop a certain medication, or an activity, for a period of time before the test will be accurate. If this is the case, you will be asked to change your behavior and return to have a sample taken at a later date. Other times, the sample for flow cytometry can be taken at the same visit where the doctor decides that the test is needed.

If your doctor wants you to have a flow cytometry test, it's important to ask if and how any medications or supplements you take might affect the test results. You should also ask what type of sample will be taken, and if you'll need to come back for a separate visit. You should also let your doctor know if you're pregnant or think you might be pregnant and if you're sensitive or allergic to any medicines or medical items (such as latex).


The amount of time it will take to complete the test will depend on what type of sample is needed. A blood sample should only take a few minutes. It will be no different than any other blood test that you get at the doctor's office. A semen sample would be expected to take longer. However, the length of time is very individual. It depends on how comfortable you are in the office and how long it takes to provide the sample.

Bone marrow tests will take longer. How much longer will depend on whether you need only local anesthetic or if you need IV sedation. If you just need local sedation, the whole process should take around half an hour. There will be 5-10 minutes of preparation, 10 minutes to do the procedure, and 10-15 minutes of recovery time. At that point, you can leave and get back to your day. If IV sedation is used, you'll probably be at the hospital for longer. Give yourself an hour or more, to be safe, and plan to have someone drive you home.

If you need another type of biopsy, the amount of time will depend on the specific type of tissue being sampled. Getting a sample from the liver is very different from getting a sample from a lymph node. Therefore, you should talk to your doctor about how to set your expectations.


Blood tests can generally be done at your doctor's office or any testing site. Some doctor's offices can also do a bone marrow biopsy on site. However, bone marrow tests and other biopsies are more often done in a hospital or other clinic setting. This is because not all doctors keep the anesthesia and other supplies on hand. Some types of biopsy may require you to go to a surgical suite.

What to Wear

If you are undergoing a bone marrow test or other biopsy, you should expect to need to change into a gown for the test. Depending on the specific type of test, you may or may not be able to keep certain items of clothing on. You may also be asked to remove jewelry, particularly if any imaging is needed.

For a blood draw, you can expect to have to roll up your sleeve, so wearing a short-sleeved shirt or one easy to push up is a good choice.

Food and Drink

In general, you should be able to eat and drink as usual before a flow cytometry test. However, this may not be true if you have to undergo certain types of sedation. If you are having a bone marrow test or other biopsy, check with your doctor about whether it is safe for you to eat and drink after midnight. In addition, it is important to talk to your doctor about whether there are any medications you will need to stop taking before your test. The answer will vary depending on what type of sample is being used, and what is being tested for.

Cost and Health Insurance

Flow cytometry can be quite expensive. Therefore, some insurers require prior authorization before these tests can be performed. This varies both by the insurer and by the indication for the procedure. It is important to check the guidelines for your specific insurer whether prior authorization is required in order for the test you need to be covered.

What to Bring

It is always a good idea to bring your health insurance card and photo identification if you need a procedure done. This includes undergoing a bone marrow test or tissue biopsy. If you will be put under sedation, you may also need to bring someone to drive you home. If you're not certain if you will be put under sedation, check with your doctor prior to the day of the appointment.

During the Test

If you are having a blood sample taken for flow cytometry, you may only interact with a phlebotomist, nurse, or technician. However, if you are having a bone marrow test or biopsy, at least one doctor will be present. In addition, there will be other team members there as well. How many individuals are present will depend on whether you need sedation and the specific type of sample being taken.

For a blood test, you will simply expose your arm and the technician will place a tourniquet, sanitize the site, insert a needle, fill one or more tubes with blood, and then bandage the site. For semen collection, you will be directed to a private room where you will masturbate and collect the sample in a sterile cup.


Before a bone marrow test or biopsy, you will be asked to change into a gown. If you are having a bone marrow test or biopsy under local anesthetic, your skin will be cleaned and the anesthetic will be injected near the biopsy site. If IV sedation is being used, an IV will be started in one of your arms. You will asked to lie in a position that gives the doctor good access to the test site. If you are uncomfortable or think you will have difficulty staying still in that position, let the doctor know. They may be able to provide you with pillows and supports to help you stay in place. Particularly for a bone marrow test, it is important to be able to lie as still as possible.

Throughout the Test

For a bone marrow test, once the anesthetic or sedation has begun to work, you will be told to lie very still. A small incision (cut) may be made over the test site. Then, a special needle will be inserted through the bone and into the bone marrow. This may be uncomfortable, and you may feel pressure. Bone marrow will be sucked up into the needle, which may cause a pulling feeling. A second needle may be used to remove a solid piece of bone marrow. When the sample has been collected, the doctor will put pressure on the site until the bleeding stops. Then a bandage will be put on the site. You will then be asked to lie still for 10-15 minutes.

If you are having a biopsy at another site, the procedure is similar. However, imaging may be used to make certain that the needle is going into the right place. For example, if they are trying to take a biopsy from a mass, they may use ultrasound to guide the needle to the right location. In certain circumstances, the sample used for flow cytometry may need to be taken under general anesthesia. If this is the case, you will be asleep for the entire procedure.


If you experience any pain or discomfort after your test, you may be told to take a pain reliever. Your doctor will likely tell you to contact her if you experience any fever after the test or any sign of inflammation or infection around where the biopsy was taken. If you needed no anesthetic, or only had local anesthetic, you should be able to get back to your daily routine. If you had IV sedation or general anesthesia, you should expect to take it easy for the rest of the day.

After the Test

It will likely take a few days, or longer, for your doctor to receive the results of your test. If you've had a blood test, there won't need to be any changes in your routine. After a bone marrow test, you'll need to avoid getting the bandage or test site wet for 24 hours. That means no bathing, showering, or swimming. You'll also be told not to engage in vigorous activity for a day or two.

If you've had a biopsy at another site, care will depend on the specific way they took the sample. You may be able to get back to normal quickly. It could also take a couple of days.

Managing Side Effects

It is important to contact your doctor if you experience any of the following after a bone marrow test or other biopsy:

  • Fever
  • Bleeding that doesn't stop or that soaks through the bandage
  • Pain that gets worse with time
  • Swelling at the site of the biopsy
  • Redness or drainage at the biopsy site that gets worse with time.

If you have mild to moderate pain after one of these procedures, it should generally be manageable with over-the-counter painkillers. However, if you have any bleeding issues, you may be instructed not to use aspirin and certain other medications. Therefore, it is a good idea to check with your doctor about what option (e.g. acetaminophen) would be best.

Interpreting Results

Flow cytometry results can take up to several weeks to come back. How long will depend on the specific types of cells that are being looked for as well as what lab is used. Flow cytometry is a very technical test, and many labs may have only limited ability to process samples.

Specific results will depend on the reason the test is being done. Information about some common types of flow cytometry test results are below.

  • Immunophenotyping is a type of flow cytometry used to diagnose leukemia or lymphoma. This test is usually done after abnormal results are seen on a complete blood count or WBC differential. With immunophenotyping, your results will state whether any abnormal cells are present and what types of cells they are. Then your doctor will use this information, in combination with information about your symptoms, to determine if you have leukemia or lymphoma. This information may also be able to determine what specific type of disease you have.
  • A reticulocyte count shows how actively your bone marrow is producing red blood cells. It is usually interpreted in combination with other test results, such as red blood cell count and/or hematocrit. The results of this test should not be interpreted on its own. Both low and high values can be fine, or problematic, depending on the circumstances.
  • CD4 counts give the number of CD4 cells in a cubic millimeter of blood. A normal CD4 count is between 500-1500. If your CD4 count is below 500, it means that your immune system may be being suppressed—by HIV or by taking drugs to suppress your immune system. For people with HIV, CD4 counts can vary with time even without your health changing. Your doctor will probably tell you about patterns in your test results, rather than looking at specific results. For people on immunosuppression drugs after an organ transplant, a low CD4 count means those drugs are working.
  • HLA test results will say whether or not an organ donor and recipient are matched and, if not, how many mismatches they have. Low numbers mean that a transplant is more likely to be successful, and a pairing with no mismatches is best.
  • Semen analysis results will tell you how many sperm are in a sample as well as how well they are functioning.

Follow Up

In most cases, if flow cytometry is being used for diagnosis, it will only be done once. However, if flow cytometry is being used to monitor a health condition, you may need to have the test repeated on a regular basis. For example, people with HIV may be recommended to undergo CD4 testing as often as every six months, or even more frequently, depending on how they are responding to medication.

A Word From Verywell

Depending on why you're undergoing flow cytometry testing, waiting for results can be nerve-wracking. Therefore, it's important to talk to your doctor about what to expect. That means asking not just how quickly you'll be likely to receive your results but what, if any, additional testing may be needed. It's possible that flow cytometry may be just the first step in understanding your health, or the definitive diagnostic step of a long workup.

No matter why you're getting flow cytometry testing, don't be afraid to seek out support. It may seem obvious that talking to someone can be helpful if you're worried about a cancer diagnosis. But it can be equally useful if you're living with HIV, or even considering being an organ donor. Sometimes, having someone to share your worries with is the best way to keep them from overwhelming your life.

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